The present invention relates to a device for detecting an electroencephalogram (EEG) and evoked response by making use of a method of monopolar derivation wherein one electrode is attached to the scalp and the other electrode mounted on a body portion other than the scalp.
According to the monopolar derivation method, a reference electrode is attached in general at a head location such as on an earlobe, the nose, the jaw, and the like. However, such a reference electrode causes some troubles to an interpretation of the result since it is sometimes activated by EEG or evoked response. To solve this problem the reference electrode is often mounted on a body portion other than a head location, whereas there arises another problem that the noise produced due to an electrocardiograph (EKG) and mixed into EEG as artifacts increases.
U.S. Pat. No. 4,421,121 is known as providing an EEG detector-unit capable of eliminating the noise caused by the EKG, wherein an EKG signal is detected by another pair of electrodes and then is subtracted from an EEG under the correlation of the amplitude with each other. However, it is difficult to eliminate with high accuracy such noises involved in the EEG because the EKG different from one mixed into the EEG is used for cancellation.
In addition, here is another problem, also upon detecting an evoked response to a stimulation, namely, an evoked response being masked with EKG noises as well as being barried in a background EEG owing to use of the monopolar derivation method. Thereupon, a method is known of eliminating the background EEG by averaging the evoked response signal in synchronization with the stimulation, but it is again difficult to eliminate noises produced due to an EKG which mixes at a high level into the EEG signal in concern.